Such materials are used in particular for insulating electric conductors which provide the standing cabling in a telephone exchange. They must meet numerous dielectric strength and mechanical performance requirements, in particular when hot, and also, in the case of fire in the telephone exchange, firstly they must be difficult to burn, i.e. have a high oxygen index, and secondly, if they do burn, they must not evolve excessively corrosive fumes. The limiting oxygen index is a known index designating the minimum oxygen content, in an oxygen-nitrogen mixture, which will maintain combustion of a material disposed vertically in said mixture when the top of said material is lit.
The insulating material is constituted by a matrix of an organic polymer with possible additives, in particular fire-proofing agents.
Up till now, the most used substances in telephone exchanges have been polyvinyl chlorides which, already by their structure, have a high oxygen index but have the disadvantage of evolving corrosive hydrochloric acid fumes which are a great hindrance. Other materials can be used, e.g. polyamides, silicones or fluorinated derivatives, but high cost limits the use thereof.
Reticulated polyethylene can also be used, but reticulation requires high investment if the material is to be extruded at high speed. Copolymers of propylene and of ethylene, high-density polyethylene and thermoplastic polyesters can also be satisfactorily used due to their mechanical properties and to their resistance to ageing when hot, but, to be used, this type of substance requires effective fire-proofing, as does the previous type. The initial limiting oxygen index of copolymers of propylene and of ethylene with a low ethylene content is indeed close to 18, but it is necessary to obtain a limiting oxygen index of 27.
Numerous fire-proofing agents are known, but the problem of resistance to fire is made complex due to the fact that very various requirements must be met at the same time and that the action of the same fire-proofing agent varies as a function of the matrix and of the other additives.
In particular, these have been used trihydrated alumina and organic substances which contain chlorine or bromine, e.g hexabromocyclododecane, tetrabromoethane or aromatic halogenated compounds.
It is also known that a generator of free radicals added in small quantities can increase the efficiency of the fire-proofing agent.
An article by Eichborn (Amer. Chem. Soc. Div. Org. Coatings Platics Chem. Preprints, 23(1), 37(1964) and J. Appl. Polymer Sci., 8,2497 (1964)) mentions the use of dicumyl peroxide with an aliphatic brominated fire-proofing agent which is tetrabromoethane in a matrix of polystyrene.
U.S. Pat. No. 3,850,882 (Underwood et al.) mentions the use of a generator of carbon-carbon free radicals, i.e. in which the molecule breaks between two carbon atoms, with numerous and various brominated fire-proofing agents which, however, all contain at least one aromatic nucleus.
It must be observed that the use of a generator of carbon-carbon free radicals with various known aromatic compounds as fire-proofing agents such as decabromodiphenyloxide and 1-2 di(2,4,6-tribromophenoxyethane) also gives bad results. What is meant here by bad result is obtaining an oxygen index lower than 22 despite the addition of an optimum proportion of antimony oxide Sb.sub.2 O.sub.3.
The present invention aims to produce a fire-proofed insulating material for an electric cable, having a high oxygen index, low chemical aggressivity of the fumes evolved in the case of combustion, good mechanical and electrical qualities, in particular when hot and when cold (coiling), good resistance to thermal ageing, while being inexpensive to manufacture.